An electric apparatus such as a motor or the like, generates heat when it is in an abnormal state and if it is continued to be used, burning of the insulating coating of the coil and other parts results, thereby making it impossible for same to be used any longer. In order to prevent this, a switch device (a protective device) is used that opens the contacts by using a bimetal that deforms at the time of an electric current overload or excessive temperature, thereby bring about an OFF state.
Examples of such switch devices are shown in FIGS. 7 through 10.
FIG. 7 shows a top plan view with the housing cut away of a switch device 21, FIG. 8 is a cross section taken along line 8--8 in FIG. 7, and FIG. 9 is an expanded partial cross section of FIG. 8.
In a container 22 made, for example, of cold-drawn steel material, there is provided a first electrically conductive plate 25 electrically separated from container 22 by an insulation sheet 30A at the bottom thereof and a second electrically conductive plate 28 disposed on and electrically separated from plate 25 by an insulation sheet 30B.
A movable contact support plate 23 is fixed as by soldering at the terminal of the first electrically conductive plate 25 and a movable contact 24 is mounted at the tip of the movable contact support plate 23. A bimetal plate 27 is fixed to the first electrically conductive plate 25 by means of a fastener 31.
The movable contact support plate 23 is provided with an aperture 23b which spacedly receives therethrough fastener 31, and the movable contact 24 is in contact with stationary contact 29 that has been mounted on the second electrically conductive plate 28.
In the condition depicted in FIGS. 8 and 9, the protrusions 23a and 23a at two locations of the movable contact support sheet 23 are separated from the bimetal plate 27 by a dimension d which is shown in FIG. 9, the movable contact 24 compressively engaging the stationary contact 29 by the inherent elasticity of the movable contact support plate 23, the first and second electrically conductive plates 25 and 28 being electrically connected with the switch device 21 in an ON state.
The terminal 25a of the first electrically conductive plate 25 is connected with a lead wire 33 and the terminal of the second electrically conductive plate 28 is connected with a lead wire 32. The terminals 25a and 28a are comprssed or clamped terminals; however, the clamp details are not shown in the drawings. The switch device 21 is installed inside an electric apparatus (such as a motor) which is not shown in the drawing and it is connected in series between the electric apparatus and the electric source.
If the electric apparatus is subjected to an overload during its usage, thereby elevating the temperature and causing the temperature of the switch device 21 to rise or if a large electric current flows, with a result that the movable contact support plate 23 is heated, the temperature of the bimetal 27 also rises and it deforms, thereby engaging the protrusions 23a and 23a of the movable contact support plate 23 and, if the deformation of the bimetal plate 23 continues, the bimetal plate 27 pushes up and movable contact support plate 23 as shown in FIG. 10, with a result that the movable contact 24 moves away from the fixed contact 29, electrically separating the first and second electrically conductive plates 25 and 28 with a result that the switch device 21 will be in an OFF state, thereby protecting the electric apparatus.
When the condition shown in FIG. 10 occurs, however, the movable contact support plate 23 and the bimetal plate 27 discharge heat, thereby lowering their temperature, and the bimetal plate 27 is restored to its original shape and the movable contact support sheet 23 moves away from the protrusions 23a and 23a, with a result that the switch device returns to the ON state shown in FIG. 9.
If the time between the OFF state of the switch device 21 and its ON state is excessively short, the switch device 21 will move between the ON and OFF states in a short cycle, which is undesirable.
Accordingly, the position of the bimetal 27 in the vertical direction is adjusted by a micro-adjustment means which is not shown in the drawing but which is provided on fastener 31 in such a manner that the distance d between the bimetal 27 and the movable contact support plate 23 may assume a preselected dimension in conformity with the curvature of the bimetal 27 and in conformity with the temperature selected for the aforementioned OFF state (such as, for example, a prescribed temperature in the range between 70 and 150 degrees centigrade). For instance, d is set at one millimeter when the selected temperature for the OFF state is 70 degrees centigrade and d is set at two millimeters when the selected temperature for the OFF state happens to be 100 degrees centigrade. However, this fine adjustment will have to be carried out manually for each switch and, as such, it is troublesome and cannot always be accurate.
If the distance d happens to be excessively large, the switch device 21 turns OFF after the electric apparatus has been left in the abnormal state for an undesirably long time, with a consequence that the protection of the electric apparatus becomes uncertain.
If the distance d happens to be excessively small, on the other hand, the cycle at which the switch device 21 moves between ON and OFF becomes too small. For example, ON and OFF states repeated at the cycle of 0.3 second (which is called chattering) is undesirable and causes fatiguing of the bimetal plate 27 or the movable contact support, thereby making it impossible to carry out normal deformation. If this occurs the device may remain in the ON state if there takes place contact deposition due to a spark, thereby making it impossible to protect the electric appliance.